Aspects of children's temperament, or biologically based differences in reactivity and regulation, are often studied in relation to symptoms of psychopathology. Specifically, children's propensity to experience negative emotions, or negative reactivity, has been related to both internalizing and externalizing symptoms (Muris, 2006). Negative reactivity can be differentiated as young as infancy into two motivational reactions to perceived threat: withdrawal, experienced as fear, and approach, experienced as frustration. Fear and frustration reactivity, thus, may differentially confer risk for either internalizing or externalizing symptoms. Evidence suggests that high fear reactivity may be related specifically to risk for internalizing symptoms, while frustration reactivity appears to bea more pervasive risk for both symptom types, though more strongly related to externalizing symptoms (Kagan, Snidman, Zentner, & Peterson, 1999; Rothbart, 2007). As opposed to the early developing reactive components of temperament, self-regulation matures later over the preschool years. One component of self-regulation, effortful control, is defined as the capacity to inhibit a dominant response in favor of a subdominant, more adaptive behavior and has been associated with reduced symptoms of psychopathology (Eisenberg et al., 2001). This regulatory component of temperament may, therefore, serve as a moderator of the relation between negative reactivity and children's symptoms. While temperament-by- temperament interactions appear very likely, particularly between reactive and regulatory components, little research has examined interactions between these factors in predicting growth of children's internalizing and externalizing symptoms over time. Further, very few studies have examined fear and frustration as independent components of negative reactivity within the same model. The proposed study will add to the existing literature by examining effortful control as a moderator of both fear and frustration on children's internalizing and externalizing symptoms to allow for differential effect of components of reactivity on symptom type. Additionally, longitudinal data will be utilized to examine how the development of effortful control may moderate transactional growth between reactivity and symptoms over time. A community based sample of 306 children and their female primary caregivers were assessed across four waves of data when the children were 3 to 5 years old. Multiple assessment methods were implemented, including physiological and observed measures, neuropsychological assessment, and mother-report questionnaire data. This sample provides a unique opportunity to examine how initial development and subsequent growth in regulatory abilities may diminish the relation between negative reactivity and the emergence and growth of children's symptoms. Results from this research have the potential to highlight specific targets for interventions that alter the trajectory of symptom growth.